In the prior art, in automatic or automated motor vehicle transmissions, mechanically driven pressure medium pumps are usually used for supplying the transmission and for controlling the shifting elements. If for the supply of the transmission and the control of the shifting elements pressure medium pumps with a constant delivery volume are used, then the delivery volume of the pressure medium pump is in a fixed ratio to the drive rotational speed of the pressure medium pump, which corresponds for example to a transmission input rotational speed if the pressure medium pump is arranged on a transmission input shaft.
The design of a hydraulic or pneumatic system supplied by a pressure medium pump with a constant delivery volume is intended to meet the demand of the largest individual consumer or, if a plurality of individual consumers are to be actuated together, then if necessary to meet the demands of them all. By virtue of the constant delivery volume of the pressure medium pump, the pressure build-up related to the drive rotational speed of the pressure medium pump is therefore predetermined for other consumers in the hydraulic or pneumatic system.
For that reason, in the prior art pressure-limiting valves are known, which open if a certain pressure in the main system is exceeded in order to divert the surplus pumped medium through a bypass. It is also known to limit the pressure of the medium being delivered to a specifiable maximum value already on the input side of the consumer points or even in the supply line to the consumer points. This is particularly necessary when the consumer points contain mechanically sensitive components which, if the maximum pressure is exceeded, would either be damaged or would wear more rapidly. Such a maximum pressure-limitation system is for example also necessary in lubricant systems in which, before the actual consumer point, an oil filter or the like is connected which should also not be acted upon by too high a pressure so as not to be damaged.
Flow valves, such as throttle valves or diaphragm valves, reduce the pressure in a system to a set value. Such valves are used in order to supply part of a hydraulic or pneumatic system with a lower pressure. Thus for example, the throttling of the fluid at the throttle point causes the pressure in a secondary circuit to be lower than in a main circuit of the hydraulic or pneumatic system.
By means of conventional pressure-limiting valves the system pressure can only be limited upward, whereas by means of flow valves the system pressure in the line after the valve can only be reduced overall. Limiting the system pressure by means of a pressure-limiting valve when the pump rotational speed is high can result in undersupplying the hydraulic or pneumatic system, whereas reducing the system pressure by means of a flow valve when the pump rotational speed is low can also result in undersupplying the hydraulic or pneumatic system. Thus, in hydraulic or pneumatic systems with a pressure medium pump with a constant delivery volume, designing the system for consumers which have to be supplied but have lower priority in the system is problematic.
From DE 40 25 578 A1 a control valve for flow or pressure regulation is known, in which the pressure medium inlet and the pressure medium outlet communicate with one another. The control piston has two control edges, one of the two control edges co-operating with a control opening and the other of the two control edges co-operating with a safety bore. Depending on the axial position of the control piston, the oil line on the inlet side communicates with the oil lines on the outlet side via a variable free cross-section of the control opening and the safety bore.